Characterization of black-powder igniter jets into a granular bed through analysis of condensed-phase product deposition and regions of penetration

Understanding the detailed ignition, flame-spreading, and combustion processes that take place inside a granular solid-propellant bed is vital for accurate interior ballistic modeling, and for the development of ballistic propulsion systems as a whole. Two experiments were conducted to investigate the penetration characteristics of igniter jets (produced by the combustion of a percussion primer and black-powder pellet) typical of those found in ballistic propulsion systems. The first experiment utilized both live and inert propellant, in conjunction with high-speed photography and photodetectors, to examine the penetration of igniter jets discharging radially into the propellant bed from two diametrically opposite vent holes. These tests show that the igniter jets fully penetrate the granular bed before any significant ignition and combustion of the granular solid propellant occurs. During the second experiment, a singular igniter jet was discharged axially into an inert granular bed, which simulated the geometric properties of live granular propellants. This setup was designed to study the region of particles coated by the condensed-phase products of the igniter jet. This information is very useful since the ignition behavior of live propellants is dependent upon both convective heating by the igniter jet as well as conductive heating by the deposited condensed-phase products. Equilibrium analysis predicts that these condensed-phase products contain potassium species. This was confirmed by collecting the inert granular particles and analyzing them with energy dispersive x-ray spectroscopy (EDS). Three correlations were developed that describe the region of the granular bed coated by the condensed-phase products from the igniter jet; these include: (1) the axial depth of penetration, (2) maximum radial penetration, and (3) the volume of the coated region. These parameters were found as functions of the Reynolds number based on the jet-orifice diameter, the pressure ratio of the igniter chamber to the ambient condition, and the ratio of particle diameter to jet-orifice diameter.